The agency's main initiative in this arena, N+3, calls for ideas that look ahead three generations into the future of aircraft design—from 2030 to 2035. NASA also has a similar program, N+2, which focuses more specifically on making airplanes more environmentally friendly in the nearer future, from 2020 to 2025. Through N+3, the aerospace agency is funding research into "green" planes that would use 70 percent less fuel than today's airliners while generating at least 75 percent less nitrous oxide (a greenhouse gas) as well as less noise pollution. These future aircraft are also expected to require less real estate and air space for takeoffs and landings, which could help alleviate air congestion and flight delays. (The N+2 initiative has a somewhat more modest fuel consumption target, a 50 percent reduction, but also shoots for a 75 percent cut in nitrous oxide.)

A number of aircraft-makers and aerospace engineers are bidding to become part of N+3. NASA has already awarded $12.4 million in contracts to Boeing, GE Aviation, Lockheed Martin and Northrop Grumman as well as the Massachusetts Institute of Technology (M.I.T.) to support their research and design efforts. NASA expects to hand out the next round of grants in April 2011.

Gas guzzlers
Fuel consumption is a major environmental concern, not to mention a financial one for airlines, typically consuming roughly one third of their budgets, according to the Air Transport Association trade group. The Federal Aviation Administration (FAA) estimates that by 2030, the U.S. fleet of commercial aircraft will consume more than 110 billion liters of fuel, up from about 68 billion liters this year, and nearly 160 million people will be flying on U.S. planes compared with the current 70.7 million, according to the agency. For this reason the FAA in June announced its own $125 million Continuous Lower Energy, Emissions and Noise (CLEEN) program to promote environmentally friendlier flight by modifying existing designs, as opposed to NASA's strategy to solicit designs for entirely new aircraft.

Commercial aircraft are a small but significant contributor to greenhouse gases in the environment because of the amount of jet fuel they burn—they account for about 2 percent of yearly carbon dioxide emissions, according to Green Aviation International Association, an eco-flight trade group based in the U.K. Airplanes generate 670 million tons of carbon dioxide each year, twice the figure from 1990. That number is projected to double by 2025, the group says.

Environmental concerns played a major role in the decision by U.K. officials earlier this year to cancel plans to add a third runway at London's Heathrow Airport, the world’s busiest international airfield.
Rethinking commercial airplanes
Aircraft-makers have been conservative about changes they make to the basic design of their planes over the past several decades because major redesigns are an expensive gamble. Designing and building a new model from the rivets up is a $10-billion proposition, says Mark Drela, professor of fluid dynamics in the M.I.T. Department of Aeronautics and Astronautics, and a leader of the university's N+3 team. "You literally bet the company on almost every new plane," he adds. Drela's estimate may be conservative, given the notorious cost overruns in the airline industry.

As a result, the configuration of the typical passenger jet has not kept pace with the more dramatic advances in engine design since the 1950s. "We are still flying aircraft whose basic template is 50-plus years old—a long tube with wings projecting outwards, engine attached to them," says Andrew Pozniak, managing director of Green Aviation, who supports NASA's initiative but questions whether any of the new designs ever will be commercially viable.

The idea behind N+3 is to rethink the possibilities. One vehicle proposed by Drela's group, which included researchers from Pratt & Whitney and Aurora Flight Sciences resembles a bulbous, flat-topped baguette capable of carrying 180 passengers. A cross section of the fuselage, which is two merged cylinders, looks like fused soap bubbles—earning it the nickname the "double bubble." Conspicuously absent are engines hanging from the wings—they are mounted instead between twin tail struts, where they provide more efficient thrust—and the wings themselves are longer, thinner and more delicate-looking than those on modern jetliners like the Boeing 737 that the "double bubble" is intended to replace. M.I.T. also submitted a design for a larger plane for international travel capable of ferrying 350 passengers.

Newer engines perform most efficiently at relatively slow speeds, Drela says. Indeed, the "double bubble" is designed to fly about 8 percent slower than today's commercial airliners. But a wider body with two aisles could allow faster loading and unloading of passengers—meaning overall trip time could be less despite the flight speed, he says.

Flying hybridsResearch into biofuels promises to reduce fuel consumption and emissions with only marginal changes to planes that have remained basically unchanged for decades. Boeing's N+3 proposal focuses on a hybrid jet fuel–electric propulsion system. Such an engine could reduce fuel consumption by 70 to 90 percent compared with typical aircraft today, Boeing spokesman Daryl Stephenson says. Such a hybrid engine could be especially useful in shortening takeoff distances, he adds, providing extra kick as the vehicle goes airborne. During takeoff and climb, the aircraft would use both jet-fueled turbines and an electric drive to provide the needed power, says Marty Bradley, the Boeing engineer in charge of the subsonic design effort. While cruising, the plane would rely on either its gas turbines or electric drive.

Northrop Grumman's N+3 approach calls for a 120-passenger plane built from ceramic composites and "smart metals" that can deform and return to their original shapes. Although the aircraft resembles a conventional jet, it is designed to take off and land on runways 1,500 meters long, compared with 2,400 meters or longer for today's jets. Northrop is hoping this shorter runway space will enable these planes to be used at smaller regional airports, relieving some traffic from large international fields.

Changes on the horizon
NASA has much work ahead of it to coax big changes out of an industry that has maintained conventional approaches to engineering for so long. What's more, achieving the meaningful reductions in emissions would require that these ecofriendly changes be widespread across the entire aviation industry—not limited to one or two aircraft-makers.

In the short term the push to develop viable biofuels, which are said to be carbon-neutral, likely will dominate efforts to make commercial aviation easier on the environment. "Already three airlines have had successful trials with a mix of kerosene and various biofuels, such as Jatropha and camelina," and fuels based on algae also have shown promise, Green Aviation's Pozniak says. Europe is pushing biofuels over the next couple of decades through emissions trading rules that require airlines to pay for their carbon use, he adds.

And the most innovative aircraft redesigns are yet to come. The truly green plane of the future will run on solar energy, requiring no fossil fuels and generating zero emissions, Pozniak says. A group called the Solar Impulse project, led by the balloonist Bertrand Piccard, successfully tested its Solar Impulse HB-SIA, on a 26-hour voyage in Switzerland earlier this month.

"We are setting the bar rather high," says Jay Dryer, director of NASA's Fundamental Aeronautics Program Office, who is overseeing N+3. "We're really trying to push as many [advances] simultaneously as we can." Dryer doubts that one design will be able to meet all of the agency's targets. More likely, certain elements of the various proposals will appeal to manufacturers and eventually make their way into airplanes, he adds. Solar flight might be the most ecologically friendly technology, but Dryer predicts it will not be mature enough to ferry large numbers of passengers for a half century or more.

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